It is common practice in the power steering art to provide steering gear mechanisms with a power assist for controlling turning maneuvers of a vehicle. The power assist is achieved by a fluid pressure motor that responds to the rotary motion of a rotary steering valve that is actuated by the vehicle operator. Differing steering characteristics can be achieved depending upon the calibration of the rotary valve mechanism.
It is known practice also to provide a steering system wherein the degree of power assist can be changed depending upon the vehicle speed. It is desirable to provide a high degree of power assist during parking maneuvers of the vehicle and during steering maneuvers at low speeds, whereas the steering assist that is desirable for high speeds is relatively reduced.
The steering valve of such known designs may be a rotary valve mechanism in which a sleeve valve member is connected through a rack and pinion mechanism to steering gear linkages for the vehicle and an inner rotary valve member is connected to the steering shaft. The valve members are provided with registering valve lands. The steering shaft is connected to the rack and pinion mechanism by means of a torsion member which permits relative angular movement of one valve member with respect to the other thereby controlling pressure distribution to the fluid motor. Torque applied to the steering shaft in one direction will develop a pressure differential in the motor in one direction for a turning maneuver in one direction and the steering torque applied to the steering shaft in the opposite direction will achieve a pressure differential in the opposite direction.
The steering gear mechanism of such known designs is powered by a power steering pump driven by the vehicle engine. The pump comprises a flow control valve that achieves a constant flow regardless of pump speed. Relative angular adjustment of the valve members then will create a steering pressure depending upon the degree of relative movement, which in turn depends upon the steering torque applied to the steering shaft.
In my co-pending patent application Ser. No. 811,963, filed Dec. 23, 1991, entitled Electronic Power Assist Control, I have disclosed a steering system in which the relationship between steering torque and steering pressure within a low torque range is different than the corresponding relationship within a high torque range. Upon a given change in torque in a torque range greater than a so-called breakpoint value, the change in steering pressure is greater than the change in steering pressure that occurs for the same torque change in a torque range below the breakpoint.
The steering gear mechanism of my co-pending application includes reaction pistons which are subjected to steering pressure so that a reaction force opposes relative movement of the valve members, one with respect to the other. A modulator valve mechanism modifies the steering pressure applied to each reaction piston to establish one or more breakpoints in the relationship between steering pressure and torque.